Looking back over the past 20 years, Janet Halley, a professor of law at Harvard University, perceives a "fierce turn in American feminism towards the state" and a powerful tendency towards "criminalising and illegalising as many of the bad things that men did to women as feminism could articulate". In the process, she believes that feminism "has lost a certain power of critical thinking", the clarity of vision that would allow it to focus on "what law really does in a complex society". Feminism, she argues, should "take a break" - and a good, long, thoughtful look at things as they are.

I want to know exactly what she means. Take a time-out, as in basketball, before returning to the fray? Or "break away from old forms of feminism altogether"? She doesn't answer directly. The phrase, she says, "tells you a lot about your personal anxieties about feminism. And I hope I can use that ambiguity to reveal how people individually feel about the centrality of feminism. The purpose of my book is to push against the idea that feminism - or any theory that the left has about power and sexuality - is somehow 'right'. I want to move the issues from that certainty to a place of hypothesis."

But feminism is a "movement", I argue. Surely, like the shark, if it stops moving forward, it sinks? "I don't think so, no. I think feminism is pretty resilient and thoroughly embedded in the world that we inhabit now. Feminism should be credited with more strength than people often give it credit for. The paranoid attitude that if we're not religiously loyal to feminism, in a devotional sense, then it will die is not a healthy attitude. It was fairly common in the 1990s in America and helped create a sense of paralysis - I attribute that disabling paralysis to the mandate that one must be feminist all the time, without a break. That's not responsive to the complexity of the world."

In her book Split Decisions: How and Why to Take a Break from Feminism, Halley distinguishes between power feminism, sex-positive feminism, cultural feminism, liberal feminism, socialist feminism and governance feminism. "It's a sign of the vitality of feminism that it hosts so many alternatives. Which is why, by the way, I think it's important to 'take a break'.

Let's say you walk into a grocery store. You find yourself among a swirling crowd of shoppers. Let's assume you see a mom who's separated from her kid. Huge crowd, lost kid. The kid's crying for its mom. At last they're reunited. But instead of being happy and relieved, the mom screams violently at the kid, maybe even slaps it. Now, how are you going to understand that interaction? I'm going to assume that this mom felt so guilty, she felt so scared and ashamed that her child was lost, that she had to behave like that - not that the mom is a child-abuser. You need hypothesis to get to that conclusion.

Now think about feminism. Historically it's developed around opposition between male and female. But maybe it's not about that. Maybe it's about old and young, maybe it's about anxiety or fear, maybe it's about something else. You need to get outside, to stand apart, to understand in an effective way what these interactions between people and positions are about."

Halley doesn't seem to buy into what one might call the wave-theory of feminism. "It's not the thing I'm most interested in, no. But I do, historically, note that there have been waves. And there are certain texts - Catharine MacKinnon's articles in the 1980s, for example, that are historically embedded in that moment, and should be regarded as classics. But we can also use them to perceive social formations which are still with us, even though that wave has passed."

So is her position essentially post-feminist? Have the battles fought since the 1960s been won, so that we can now enjoy the luxury of internal debate, dispute and disagreement? It turns out that she is talking specifically about a particular kind of American feminism. "There are still places where male domination has a very familiar, structural and immobile character and I think we need feminism to help us with that."

Which leads me to a fairly obvious question. She is an eminent female professor at Harvard. That university was at the centre of a storm last year, with its (recently resigned) president's reported pronouncements about women's genetic handicaps where science was concerned. Is that partly what she is thinking about?

"Larry Summers lost his job. They brought down one of the most powerful men in the American academy. I think that the people who wield that feminist power should admit to it, and come to terms with the fact that they have it."

So where does feminism go, post-Halley's break? "I'm in the break myself at the moment. I'm writing about things that have nothing to do with feminism. I think this is very common. What I'm trying to do is draw attention to the political possibilities of multiplicity, the fact that one has a bunch of different gears one can slip into. I'm a leftist, for example. And I care just as much about that as feminism. I would also note, in passing, that feminism is not necessarily friendly to leftism - they don't necessarily work well together. You need to have a contingent attitude about both affiliations, without being religiously aligned with one or the other, keeping them both in play. Feminism, I believe, can have that optional character without becoming dead or paralysed."

Does she still feel a current of strong optimism in feminism, that optimism about possibilities which energised the movement's earlier phases? "I think at a time like now when three wars are going on in the Middle East it feels like a very serious time for us all politically and it's hard to talk about optimism."

· Janet Halley's Split Decisions: How and Why to Take a Break from Feminism, Princeton University Press